Carbonaceous materials have been particularly popular owing to their large surface area, ease of preparation from a wide range of sources, such as coconut shell, wood powder, peat, bone, coal tar, resin and resorcinol formaldehyde and related polymers, to yield active material with a Brunauer-Emmet-Teller (BET) surface area ranging between 1000-1500 m2/g. The resultant carbon has several superior properties including fast adsorption, especially in the case of large amount of gas molecules and ions in solution to enable their advantages during its application in electrochemical double layer capacitor, fuel cell electrodes and hydrogen storage. Among available resources, coconut shells provide an attractive source for the preparation of carbon with high surface area and also being an easily available raw material, the process becomes cost effective and environmentally sustainable. This activated carbon is highly pure with uniform pore size distribution, which makes them suitable for a variety of applications.
The preparation of high surface area carbon from coconut shell involves a sequence of procedures involving two major steps viz. carbonization and activation. Initially the coconut shells were treated with chemical activating agents for dehydration followed by prolonged pyrolysis to form carbonaceous materials, where these activating agents also help to reduce the formation of tar and other byproducts, thereby increasing the yield. The most widely used activating agents are alkali metal hydroxides, carbonates, chlorides, sulfides, thiocyanate and also inorganic acids. The carbonization step includes thermal decomposition of the coconut shell material, which eliminates non-carbon species producing a fixed carbon mass with random pore structure. Very fine and closed pores are created during this step. The next step involves the activation of carbonized sample. The purpose of activation is to open up and enlarge the existing closed pores by thermal treatment under a dynamic flow of gases such as carbon dioxide, steam, nitrogen, ammonia, and halogens depending upon the nature and application of the material. These gases in turn react with the surface functional groups to make them either acidic or basic and the weight loss during this activation is monitored to have an idea about the porosity change.
Reference is made to Microporous and Mesoporous Materials 27(1999) 11-18 wherein in the conversion of coconut shell to high surface area carbon using KOH in various ratios between the temperature range 600-800° C. has been extensively reported. Reference is also made to European Patent EP 649815 A1 26 Apr. 1995 wherein the carbon prepared by alkali solution treatment in the temperature range 800-1100° C. gives fine powder with highly microporous structure with average pore diameter 8-20 Å has been reported. The drawbacks of such methods are formation of potassium compounds that are unstable in the atmosphere of CO2, O2 and H2O moisture induces the changes in the micro structure, changes in the surface functional groups upon storage and high resistance due to carbonate formation. These carbonates also can clog at least some pores thus deleteriously affecting the performance. Moreover, the need of high temperature (700-1100° C.) limits the use of such reagents for efficient activation and compounds can be reduced to metallic potassium by carbon at this temperature. Reference is made to U.S. Pat. No. 6,013,208 Nakamura, et al. Jan. 11, 2000, wherein another manufacturing method for preparing carbonaceous material for electrical double layer capacitor from lignocellulosic material by using gases like halogen, steam and hydrogen at higher temperature range (800-1000° C.) is disclosed. The main drawback is that the presence of micro pores with attendant non-uniformity can cause too much of variation. Also, the high temperature halogen gas treatment makes the process corrosive and hazardous. Initially making halogenated carbon and again dehalogenating, makes the process time consuming and costly. Reference is made to Chinese Patent No. 25(2), 247-251 1997, wherein another important class of carbonaceous materials, the carbon areogels, is typically prepared from resorcinol-formaldehyde gels by supercritical drying and carbonization at higher temperatures <1000° C. The double layer capacitance was found to be 30 F/g. The main drawback of this work is that the supercritical method of drying makes the process hazardous and expensive for practical applications. Another limitation is that the high-pressure experimental set up is always cumbersome both to maintain and to use. Reference is also be made to Carbon 39(2001) 877-886, wherein the zinc chloride activation is carried out at high temperature (800-1000° C.) using the gases N2, CO2 and steam. The drawback of using steam is that it is very difficult to regulate the process due to uncontrollable kinetics.
Reference is made to GB Patent 435345, wherein improvements in or relating to the manufacture of activated carbon from the black ash residue from esparto grass using steam at 700-1000° C. have been reported. The drawback of using steam at high temperature is that it is very difficult to regulate the process due to uncontrolled kinetics. Reference is be made to GB Patent 965709, wherein the carbon prepared from anthracite using steam at 800-900° C. is reported. The surface area is very low (280 m2/g) and the primary source of carbon is anthracite and more significantly, low surface area can give poor charge storage capability. Reference is also be made to GB Patent 1094914, wherein a process for the manufacture of porous carbon electrode for use in the air depolarized cell has been reported from the admixture of acetylene black, activated vegetable (coconut shell) carbon and polymethylacrylate as a binder. The invention does not describe activation parameters and the capacitance of the prepared carbon samples will be inferior as the binder may give high resistance. Reference is also made to EP 12111702 A1, wherein the activated carbon prepared from coconut shell using steam as a activating gas at 900-950° C. The surface area is in the range 2000-2500 m2/g. The main drawback is that the capacitance is very low (0.8-1 F) and the activation of coconut shell is carried out at high temperature (930° C.) using steam. The high resistance of this material will not allow fast charge delivery and hence the response time will be short.